1. Field of the Invention
A circuit for connection between the conductors of a subscriber's line via the hook contact and in parallel with the further portion of a subscriber's set, the circuit having a relatively low resistance to currents of less than a predetermined value and a relatively high resistance to currents greater than the predetermined value.
Such a circuit may be used in telephony systems. Testing by a telephone exchange as to whether or not the hook contact in a subscriber's loop is closed can be effected in a first manner by applying a voltage of a fixed value between the two wires of the loop an then measuring the current through that loop. Alternatively, testing may be effected in a second manner by passing a current of a fixed value through the loop and measuring the voltage across the loop. In these testing modes the hook contact is assumed to be closed if when testing in the first manner the current exceeds a certain predetermined value (denoted the minimal detection current hereinafter) or if when testing in the second manner the voltage remains below a predetermined value (denoted the maximum detection voltage hereinafter). In the first testing mode the central supply source may be connected during testing to the subscriber's line via supply resistors having a high value with respect to the value of the supply resistors through which the connection is made in the speech condition. An electronic subscriber's set incorporated in this loop has a high resistance if a small current is passed through it. Because of the presence of high-ohmic supply resistors and the high set resistance, it is possible that in the first-mentioned testing mode, when the hook contact is in the closed condition the current in the subscriber's loop is lower than the minimum detection current, so that the exchange does not recognise the subscriber's call. In the second testing mode the detection is prevented because of the fact that the voltage across the subscriber's loop in which the electronic portion of the set is incorporated, is higher than the maximum detection voltage, and also causes the switch closure not to be detected.
2. Description of the Prior Art
It is known to solve the above-described detection problems by using a circuit which is connected behind the hook contact and in parallel with the electronic portion of the set and which has a comparatively low resistance at direct currents below a predetermined value and a relatively high resistance at larger direct currents.
A circuit having these properties is disclosed in the U.K. Pat. No. 1454157, but this circuit has the disadvantage that it has a very complicated structure, so that it is expensive to produce and its reliability of operation is limited by its complicated structure.